Isolation and identification of the causative factors responsible for color fixation in rice bran oil

To understand the chemical nature of the dark coloring constituents responsible for color fixation in rice bran oil, crude and dewaxed rice bran oils of 6.8% free fatty acids were fractionated on a silica gel column to get a dark-colored material (0.57% of the oil). Thin-layer chromatographic analysis of the material showed a spot corresponding to monoglycerides, but there were no spots corresponding to other glycerides. It contained traces of phosphorus (<0.1 ppm, which is equivalent to 2.5 ppm phospholipids) and iron (1.3 ppm) that could not be attributed to phospholipids or to any iron-complex. Upon saponification it yielded 12% nonsaponifiable matter. Gas-liquid chromatographic analysis of the saponifiable matter (after acidification and methylation of fatty acids) showed the presence of palamitic, oleic and linoleic acids. Further, on the basis of comparison with spectroscopic data of synthetic monoglyceride, the constituent was characterized to be a mixture of monoglycerides with side chains of oxidized unsaturated fatty acids.